This invention relates to a travel-distance signal generator for a vehicle, e.g., a taxi meter or the like, which is adapted to produce a travel-distance signal for each predetermined distance of travel of the vehicle.
As travel-distance signal generators for vehicles, there have been mainly used or proposed signal generators forming a portion of an electromagnetic induction system, a variable reactance system or a photoelectric system. In the signal generator of the electromagnetic induction system, a magnet is rotated in proportion to the rotation of a wheel of the vehicle, a detecting coil is disposed near the magnet, and the magnetic flux of the magnet crosses the detecting coil a predetermined number of times with one rotation of the magnet to induce one pulse voltage in the detecting coil for each predetermined distance of travel, for example, 50 cm. The number of pulse voltages thus induced in the detecting coil is in proportion to the distance of travel of the vehicle, and the frequency of the induced pulse voltages is in proportion to the speed of the vehicle. In the signal generator of the variable reactance system, an electromagnetic shield plate is interposed between detecting coils formed by two electromagnetically coupled coils, and in proportion to the travelling speed of the vehicle, the electromagnetic shield plate is rotated to repeatedly turn on and off the electromagnetic coupling of the two coils. As a result of this, one pulse voltage is derived from the detecting coil for each predetermined distance of travel of the vehicle.
The signal generators of the electromagnetic system and the variable reactance system are incapable of accurately detecting variations in the magnetic flux or the electromagnetic coupling state when the travelling speed of the vehicle is reduced to some extent. The signal generator of the photoelectric system is free from this defect. In the photoelectric system, a light emitting element and a photo detector are disposed opposite to each other so that the latter may receive light emitted from the former, and a light shield plate which rotates in proportion to the travel of the vehicle is repeatedly interposed between the light emitting element and the photo detector to derive one electrical pulse from the photo detector for each predetermined distance of travel of the vehicle. In this conventional signal generator of the photoelectric system, however, the converted electrical signal is applied via a signal line to a taxi meter, speedometer, tachometer or like instrument as is the case with the signal generators of the electromagnetic induction system and the variable reactance system, and a defect common to these prior art signal generators is that induced noise is liable to appear on the signal line; especially, in vehicles, there are many sources of pulse-like noise waves, such as an engine ignition device and so forth, and these pulse noises are likely to be induced in the signal line. It is difficult to sufficiently remove these noises by a waveform shaping circuit and there is a risk that the noise will be processed as a distance signal. Further, where the signal generator is installed in an engine compartment of the vehicle, since the temperature in the engine compartment becomes appreciably high, there are possibilities that the light emitting element and the photo detector of the signal generator of the photoelectric system will be markedly degraded and reduced in efficiency, resulting in an unsatisfactory performance being achieved. Under such high temperature conditions, the light emitting element and the photo detector become short-lived, and the operation of replacing them with new ones is very troublesome.
It is an object of the present invention to provide a photoelectric travel-distance signal generator for a vehicle which is insusceptible to the influence of a pulse-like noise generated in the vehicle and hence is capable of producing an accurate travel-distance signal.
Another object of the present invention is to provide a photoelectric travel-distance signal generator for a vehicle which is hardly degraded, long-lived and stably operates even if installed under relatively high temperature conditions.
Another object of the present invention is to provide a photoelectric travel-distance signal generator for a vehicle which is capable of attaining the abovesaid objects, easy to manufacture and little in dispersion in performance and which is able to produce a signal of high SN ratio.
Yet another object of the present invention is to provide a photoelectric travel-distance signal generator for a vehicle which produces, with a relatively simple structure, a distance signal in response to driving of the vehicle in one direction only.